This invention generally relates to optical sensors, and more specifically, to high resolution optical sensors such as staring sensors.
A staring optical sensor is usually a two-dimensional focal plane mosaic of photodetectors, each of which generates an electric current representing the intensity of light incident on a given area of observation, referred to as a pixel and representing one unit of resolution. Typically, the area behind each photodetector is used for electronic circuitry to amplify and multiplex the detector signal.
The coverage provided by a sensor is, generally, a function of the distance between neighboring detectors the smaller that distance, the better the coverage provided by the sensor. Ideally, the photodetector elements of an optical sensor are located immediately adjacent to each other. This forms an array of detectors with minimal space between the detectors, so that the array approaches 100% coverage of the image incident on the array.
The resolution of a staring optical sensor is, generally, a function of the distance between centers of adjacent detectors, referred to as the center to center spacing. The smaller this distance, the higher the resolution of the sensor. Thus, in a high resolution, high coverage sensor, the photodetectors are quite small and are located very close to each other. Under these circumstances, the space behind each detector for the required electronic circuitry is also very small, often resulting in the use of less than optimal electronic circuitry.
Increasing the spacing between detectors while keeping the size of the individual detectors fixed, increases the available area or space for the required electronic circuitry, but this also introduces undesirable gaps between detectors. With a sensor having such gaps, the image of a target of interest that falls within a gap between detectors would be invisible to the sensor until that image moved onto one of the detectors. The larger spacing between detector centers also reduces the spatial resolution of the sensor.
In the past, various approaches have been used to build high resolution, high coverage staring focal planes. One approach has been, simply, to keep the detector size large enough to accommodate an optimally functioning electronic circuitry. With another approach, higher density integrated circuits or lower functioning circuits are used to minimize the circuit area needed, allowing the use of smaller detectors. Either approach compromises the overall performance of the sensor.